Safety interlock systems for electrical devices are well known. Typically, they are found in devices that have high voltage components, including stepper motors, such as commercial material handling devices. Service or access doors on such devices will normally actuate an interlock switch which triggers the safety interlock system. When the door is closed, the contacts of the interlock switch are likewise closed, placing the safety interlock system in an “operating” or safe mode, wherein the motor power supply is connected to the device's stepper motors. When the door is opened (such as for repairing the device), the contacts of the interlock switch are open, which triggers the safety interlock system. When the safety interlock system is triggered, the stepper motor power supply is automatically interrupted to prevent a hazardous or energized condition.
Material handling devices have in the past used mechanical switches or relays to interrupt a motor's power supply. These mechanical switches and relays basically connect the various stepper motors housed in the device to the appropriate motor power supply source(s) when a start switch is actuated, and continue the connection until actuating power is interrupted. Once power is interrupted (whether deliberately by an “off” switch or unintentionally because of power failure), the contacts of the switch or relay open and electrically disconnect the stepper motors from the motor power supply sources until the device is restarted by an operator or a service technician.
More recently, with the use of higher current motors, and larger systems with more motors, larger switches and relays have been employed. This creates additional costs. Further, because of the mechanical nature of such switches and relays, high inrush and surge currents can occur when reconnecting high current motor power sources to such high current motor circuits. These high inrush and surge currents can damage switch and relay contacts as well as other electrical elements along the motor power source pathways. This can lead to breakdowns, reliability problems, and safety concerns.
There has thus arisen the need for an interlock system which interrupts the normal functioning of a stepper motor during an open-door condition without employing mechanical switches or relays to cut off the main motor power supply. It has, therefore, been proposed to minimize the risks noted above by providing a safety interlock system which interrupts the lower voltage/current stepper motor control signals instead of the motor power supply.
When triggered by an open-door condition, the safety interlock system of the invention interrupts the sequential stepper motor control signals used by the stepper motor drive circuits instead of the high current/high voltage motor power supply circuitry as seen in the prior art. Redundant interlock logic gates are employed to interrupt incoming stepper motor control signals in the absence of an “enable” signal. Thus, without the required sequential stepper motor control signals, the stepper motors cannot rotate or will rotate at a significantly reduced, and therefore safe, torque level.